Recently, in electric equipment such as a personal digital assistant equipment represented by a notebook-sized personal computer (notebook PC), there has been a strong demand on the battery for thinning, saving weight, giving a large capacity as well as increasing the power consumption. The increase in the power consumption has caused a problem of heat generation in the battery pack constructed to be attachable to electric equipment and enclosing a battery.
In the conventional control method, only the temperature of the battery cell comprised of a nickel-cadmium battery (NiCd battery), nickel-hydrogen battery, lithium-ion battery, or lithium-polymer battery in the battery pack is detected, and the detected temperature is directly sent to the system unit. In the system unit having received the detected temperature, an approach has been taken, in which before a dangerous condition is reached, for instance, in a notebook PC, it is put in a suspend state (the power is cut while supplying power only to minimum hardware required for holding data, such as the CPU and memory, and keeping the program execution state) or the like, thereby to reduce the supply of power for keeping the battery cell at a specific temperature or lower.
Further, recently, the FET (Field Effect Transistor) is used in as protection circuits for battery cells such as lithium-ion battery cells.
FIG. 7(a) and FIG. 7(b) are figures for explaining the structure of the FET as the protection circuit. As shown in FIG. 7(a), for instance, there are provided a discharge stopping FET 201 and a charge stopping FET 202 for the battery cell as a protection circuit, and the protection of the battery cell is provided under control from a CPU 203. That is, discharging stops when the discharging stop FET 201 is turned off, and charging stops when the charge FET 202 is turned off, thereby enabling overcurrent, overcharging, and over-discharging for the battery cell to be prevented. In FIG. 7(a), the discharge stop FET 201 is made N-channel and the charge stop FET 202 is made P-channel, but they may both be made N-channel or P-channel.
In this case, if an FET as such protection circuit is provided, since a large current flows through the power line to which the FET is related, overheating of the FET has become a new problem. For instance, in the large-capacity DC/DC converter designed in a notebook PC, for instance, the input voltage (Vin) is about 16 V from the AC adapter or about 10 V from the battery voltage, and for instance, an output voltage (Vout) of about 1.6 V is supplied to the CPU on the system side (system unit CPU). In recent years, however, the load power of the CPU is tending to increase (for instance, 15 A to 20 A), causing a serious problem that the temperature of the FET becomes very high. Specifically, in the running of a special program for which the CPU operates at high speed, the temperature of the FET rises significantly and exceeds the absolute rated temperature, and as a result, the FET can be broken.
To handle such problems, for instance, as shown in FIG. 7(b), it is possible to take a measure in which two discharge stopping FETs 201 and charge stopping FETs 202 are provided in parallel, respectively. Since heating is effective in proportion to the square of the current flowing through the FET, the current flowing through one FET is halved by providing the FETs in parallel, thereby enabling the heating for one FET to be drastically reduced. That is, as shown in FIG. 7(b), by providing the respective FETs in parallel and in a plural number, it is possible to balance the current value and the temperature.